I repeated each separate experiment three times in order to eliminate any experimental errors, so I think the conclusions I have drawn are valid. The data was reliable, because I repeated the experiments.
In order to reduce the amount of background radiation reaching the infrared detector, I covered the IR emitter and detector along with the jelly under a cardboard box. This meant that the infrared detector could not be directly affected by changes in the intensity of sunlight outside or by changes in the temperature of the room.
However, one of the limitations of using this method, was that the cardboard box may have absorbed heat energy and may have emitted this in the form of infrared radiation. This would mean that the voltage across the infrared detector would have been affected by how hot the surrounding environment was, as this affects the infrared radiation emitted by the box.
Another significant limitation in the experiment was that the length and width of the jelly was not uniform, because the jelly was difficult to cut. This could have been improved by setting the jelly in a mould which has exactly the right dimensions needed. This would ensure that the length and width of the jelly is uniform.
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